The present invention relates generally to radio frequency (R.F.) receivers and more particularly to R.F. receivers adapted to process received R.F. pulses and reject received R.F. continuous wave (CW) signals.
As is known in the art a CW signal is a signal with a time duration substantially longer than the time duration of a single pulse from a pulsed signal. In R.F. receivers adapted to process received R.F. pulses, strong CW signals may interfere with the reception of the desired pulsed signals, especially if the frequencies of the CW signals are close in frequency to the carrier frequencies of the pulsed signals. For this reason, it is often desirable to filter out strong CW signals.
One method for filtering out, or tuning out a CW signal, while allowing pulsed signals to pass through for processing, is to provide a notch, or band reject filter whose center frequency is tuned to the frequency of the CW signal. Thus all received signals except those having a frequency within the notch of the band reject filter are passed for processing. Where multiple CW signals are present in the frequency band of interest, it is common for only one band reject filter to be used. This one band reject filter is tuned to filter out the strongest CW signal present in the frequency band of interest. Often, a YIG (yittrium-iron-garnet) filter is used as the band reject filter because it can be used at frequencies in the GHz range, and its band reject center frequency can be tuned over a very broad frequency range.
With such method, the reject band of the band reject filter is first tuned below the lowest frequency in a predetermined band of interest so that all frequencies within the predetermined frequency band pass to a CW signal detector. The CW signal detector determines if a received signal is a CW signal by comparing its time duration with a predetermined time duration threshold. If the time duration of the received signal exceeds the threshold, then the received signal is characterized as a CW signal. The YIG control circuit then tunes the center frequency of the band reject filter to a frequency which is slightly below the frequency band of interest. The center frequency of the band reject filter is then increased in successive steps of frequency so as to sweep through the frequency band of interest. After each step, the amplitude of the band reject filter output signal is compared to the value of a variable which is equal to the smallest amplitude of the band reject filter output signal previously measured during the present sweep. If the amplitude of the band reject filter output signal is larger than the value of the variable, then this amplitude is discarded. If the amplitude of the band reject filter output signal is less than the value of the variable, then the variable takes on a new value which is equal to the amplitude of the band reject filter output signal. In addition, the YIG control circuit stores the center frequency of the band reject filter associated with the lowest amplitude previously measured during the sweep. After the YIG control circuit has finished the sweep, the center frequency of the band reject filter is set at the frequency where the minimum amplitude occurred, i.e. the frequency where maximum CW signal rejection occurred. The band reject filter now provides maximum rejection of the strongest CW signal present within the frequency band of interest.
When the CW signal is no longer present within the frequency band of interest, the reject band of the band reject filter is returned to a frequency outside of the frequency band of interest. This allows all frequencies within the frequency band of interest to pass through the filter for processing. Since the CW signal is being rejected by the band reject filter, there will be very small change in the filter output when the CW signal is no longer present. This small change in the filter output may be too small for the post-filter circuitry to detect the absence of the CW signal with the required accuracy. Therefore, to obtain more accurate CW signal "absence" detection, the center frequency of the band reject filter is periodically tuned to another frequency away from the frequency of the previously detected CW signal, in order that the post-filter circuitry is able to measure the amplitude of this CW signal and determine if this CW signal is still present.
While such method may be useful in some applications, the time required to periodically tune the center frequency of the band reject filter away from the frequency of the previously detected CW signal to determine if this CW signal is still present may be excessive. A second problem with such method is that the periodic tuning of the center frequency of the band reject filter to a frequency away from that of the previously detected CW signal may cause distortion to the amplitudes of the pulses of interest. Additionally, when a pulse, with a carrier frequency at, or near, the frequency of the CW signal occurs during the sweep process and the center frequency of the filter is at the frequency of the CW signal, a false amplitude reading may occur. If this false amplitude is larger than any of the other amplitudes measured during the sweep, then the center frequency of the band reject filter will be tuned to an incorrect frequency. That is, the center frequency of the band reject filter will be tuned to a frequency other than that frequency required to provide maximum rejection of the CW signal.